1. Field of the Invention
This invention relates generally to tuner circuits in data communications systems. In particular, the present invention relates to a wide-band digital tuner in a communications satellite.
2. Description of Related Art
A tuner, in general, is capable of down converting from any frequency within a specified frequency range. The tuners in satellite data transmission systems are usually analog or digital with a single serial input received from the output of an analog-to-digital converter (ADC).
The basic structure of a prior art tuner used in a communications satellite is illustrated in the block diagram of FIG. 1. An output signal IF.sub.in which has been shifted down in frequency to an intermediate frequency, is applied to analog to digital converter (ADC) 12 which digitizes the signal IF.sub.in into an output containing a large number of samples at a high rate.
The tuner 14 receives the digitized signal IF.sub.in outputted by the analog to digital converter 12 and processes it into quadrature signal processing paths 16 and 18 which each contain a frequency converter 20 which downwardly shifts the input signal IF.sub.in to a lower frequency. The frequency converters 20 of the I signal processing path 16 and the Q signal processing path 18 respectively receive input carriers COS(.omega.T) and SIN(.omega.T) from the quadrature digital sinewave generator which cause the frequency converters to produce the quadrature I and Q signals which are downshifted in frequency to a lower carrier frequency. The input to the quadrature digital sinewave generator 22 is a frequency command F.sub.in which commands the quadrature digital sinewave generator 22 to output the quadrature carriers COS(.omega.T) and SIN(.omega.T) of the appropriate frequency to cause the frequency converters to shift the input signal IF.sub.in to the lower carrier frequency for further signal processing. The envelopes of the lower frequency quadrature carriers produced by connection of COS(.omega.T) and SIN(.omega.T) to the frequency converters 20 are modulated with the quadrature components of data present in the intermediate frequency input signal IF.sub.in. The outputs from the frequency converters 20 are applied to suitable low pass filters 24 which attenuate frequency components outside the desired lower carrier frequency band to which the I and Q data components are shifted. (Alternatively, tuner 14 could utilize bandpass filters instead of lowpass filters 24.) The output I and Q signals are applied to downstream demodulator processing 26 of a conventional nature including channelization, discrete Fourier transformation (DFT) and other known signal processing techniques.
The bandwidth of tuner 14 is limited and switching between multiple IF frequencies is typically accomplished before the payload is converted in ADC 12 to the single serial digital signal provided to tuner 14. These multiple IF frequencies may, but need not, be located at uniformly-spaced positions in frequency. The process of switching between different IF frequencies is sometimes referred to as "hopping". For security purposes, hopping is often performed in a quick random pattern known only to the transmitting and receiving stations. Analog components impart the disadvantages of signal attenuation, heightened noise and lower reliability. These problems are exacerbated when the data transmission system utilizes frequency hopping among different channels.
Most tuners do not operate well at the high rates associated with wideband signals in the gigabit per second range. Channelizers, in particular, are commonly used to separate an input wideband signal of a specific spectrum received from an antennae into a plurality of narrowband channels. The wideband signal may carry different channels using different frequency bands, different time slots, different spread spectrum coding, or a combination of any two or more of these techniques. The channelizers may be wideband channelizers and/or narrowband channelizers used to separate a wideband signal into smaller sections of constituent channels.
The channel distribution in a wideband satellite communications application may be a plurality of 2 GHz channel groups, each comprised of twenty (20) 100 MHz sub-band channels, or 320 MHz channel groups, each comprised of four (4) 80 MHz sub-band channels. Each sub-band channel in these implementations may be channelized into smaller sections of narrower band channels, such as, for example, four (4) narrowband channels of 20 MHz each. Of course, neither the wideband nor the narrowband signals are limited to any particular spectral range and may separately determined for different applications.